I have a few friends not from real life, but I know them from internet.
I could say I know them from this platform for many years.
But when we need help we help each other.
When you realize that you do something for better goods and you don't need anything in return at least you are fine with yourself.
I had in my life many fake friends and I not keep them close to me I can talk with them but not for long.
Is not always about money when you need help, sometimes is easy for them and hard to you to understand something or to check something or just need verify some things, when you are uncertain.

You are still young and I think you will have many situations like that, people like your classmate know that without your help they can't pass the test, but when they need to help you they risk that they get caught by teacher. So simple selfish.